This invention relates to sheet metal duct work fittings of the type used in heating, ventilating and air conditioning systems. Generally, in systems of this type, the main air supply is conveyed through sheet metal duct work from a central source. A main distribution duct is provided, which at spaced intervals includes take-off fittings through which branch lines are supplied from the main duct. The main conduit typically is rectangular and has a constant cross sectional area along its length. No compensation is made for the air volume carried therein. Outlets near the source accordingly provide an extremely high air flow while those remote from the source provide a considerably reduced air flow.
The prior art has recognized the advantages of a main conduit or trunk duct having reductions in cross sectional area along its length to provide generally equal air flow at each outlet. Such systems are however, relatively expensive and accordingly have not found commercial acceptance. Partially prefabricated air duct fittings have been proposed for use in heating and ventilating systems, however, in each case a great number of separate component parts are required including top and bottom panels, take-off sections, side panels and the like. In some of prior art systems the top and bottom panels and the side walls are provided with spaced longitudinally extending marginal tabs and corresponding slots. The tabs, when inserted in the slots are bent over to secure the plate and wall together in a rigid assembly. Such systems, however, require careful handling since the tabs are easily deformed during such handling. Additionally, if an ar tight seal is required, the tabs and slot must be sealed with putty, or similar sealing substances.
In another system known in the prior art, prefabricated assembled components of the duct system are provided. Each component is reduced in cross sectional size from the previous adjacent component whereby they may be nested for shipment and storage. In the above systems, however, separate provision must be made for the connection of take-off ducts and other fittings to registers or wall stacks or the like. In these cases either elaborate tab and slot combinations must be prefabricated or otherwise provided or in the case of the nestable preassembly an opening must be cut in the duct work in each position where it is desired to connect the take-off section.